Method for cooling a gaseous mixture to a low temperature

ABSTRACT

.Iadd. .Iadd.8. A method for cooling a second gaseous mixture to low temperature and producing at least one constituent of said mixture in liquid phase, comprising: 
     a. cooling and subjecting first gaseous mixture containing at least one component of said secong gaseous mixture, to a fractionate condensation under a first pressure, 
     b. expanding at least two condensed fractions obtained during the fractionate condensation of said first gaseous mixture, reuniting the expanded condensed fractions with said first gaseous mixture under a second pressure lower than said first pressure, vaporizing and reheating the reunited fractions with said first gaseous mixture under said second pressure, by heat exchange with said second gaseous mixture undergoing cooling, and with said first gaseous mixture undergoing fractionate condensation, and recompressing said first gaseous mixture to said first pressure, 
     c. reuniting under said first pressure said second gaseous mixture undergoing cooling with said first gaseous mixture undergoing fractionate condensation, when said gaseous mixture is in conditions of temperature and pressure such that a major portion of said constituent to be produced in liquid phase is condensed within said second gaseous mixture, and after the first condensed fraction of said first gaseous mixture has been withdrawn for expansion and vaporization, continuing the fractionate condensation of the mixture so obtained under said first pressure until there is obtained a last condensed fraction containing a major part of said constituent to be produced in liquid phase, 
     d. expanding to said second pressure the last condensed fraction, and separating the last expanded fraction into a liquid portion expanded to a pressure lower than said second pressure and a residual gaseous portion for recompression with said first gaseous mixture, and withdrawing said last expanded liquid portion as a product stream, and 
     e. prior to reuniting said first gaseous mixture and said second gaseous mixture, condensing at least partially said second gaseous mixture under a third pressure which is higher than said first pressure, and expanding said second gaseous mixture at least partially condensed to said first pressure. .Iaddend.

This is a continuation of application Ser. No. 273,338, filed July 19,1972.

The invention relates to a method for cooling a gaseous mixture to lowtemperatures and producing at least one constituent of this mixture inthe liquid state, wherein the gaseous mixture is subjected to afractionate condensation, and the constituent to be produced in theliquid state, after its condensation, is expanded to the pressure atwhich it is to be produced, then is wholly withdrawn, while at least oneof the condensed fractions is expanded and vaporized in heat exchangewith the gaseous mixture undergoing fractionate condensation, under ahigher pressure than the pressure of withdrawal of the constituent to beproduced in the liquid state, then recompressed and reunited to thegaseous mixture to be cooled, characterised in that said gaseous mixtureis used as a frigorific fluid within a cooling cycle providing thecondensation of a second gaseous mixture consisting of the same maincomponents, and during its cooling is not added to said second gaseousmixture until at a temperature level low enough so that the constituentto be produced in the liquid state be at least mostly condensed withinsaid second gaseous mixture.

It further comprises the following features, either individually or inany combination:

(1) If the gaseous mixture is available at a comparatively highpressure, the vaporized fraction is recompressed only to a lowerpressure than that of the gaseous mixture, then cooled independentlythereof, and is added to the latter only at a temperature level at whichthe constituent of the gaseous mixture of gases to be separated in theliquid state is already liquefied at least for its major portion;

(2) The gaseous mixture is used as refrigerating fluid passing through aclosed cycle to ensure the fractionated condensation of another gaseousmixture comprising the same main constituents, wherein all the expandedand vaporized fractions coming from the first mixture of gases arereunited so as to reconstitute the first mixture of gases after theirreheating and recompression;

(3) At least a part of a fraction separated during the fractionatedcondensation of the other gaseous mixture is reunited with the fractionexpanded during the vaporizing of the first gaseous mixture so as tocompensate for losses in the circuit of the latter and/or to adjust itscomposition;

(4) At least one part of a fraction separated during the fractionatedcondensation of the first mixture of gases, acting as refrigeratingfluid, is combined with a fraction separated from the other mixture ofgases so as to adjust the composition of the first mixture.

The improvement of the invention, and especially that according to (1),makes it possible to use better the cold output resulting from the freeexpansion of the gaseous mixture of which one constituent is to beproduced in the liquid state, without, however, effecting the recyclingof the fraction or fractions revaporized and recompressed under apressure which is economically too high.

The improvement according to (2) facilitates the control of aninstallation for carrying out the method and provides smaller heatexchange surfaces. Combined with the variations according to (3) andpossibly (4), it makes it possible to adapt the composition of therecycled gas to that of the gaseous mixture, of which one constituent isto be produced in the liquid state, in the case in which a gaseousmixture of variable composition is being treated.

The invention will be further described, by way of non-limitativeexample with reference to the accompanying drawings, showing twoinstallations for liquefying natural gas and comprising the improvementsdefined above.

FIG. 1 shows an installation for liquefying natural gas in which thelatter, available at a comparatively high pressure, is expanded to apressure equal to that of the recycled gas and is added thereto onlyafter the major part of the methane it contains has been liquefied.

FIG. 2 shows an installation for liquefying natural gas in which therecycled gas passes through a closed circuit which is substantiallyindependent of the natural gas circuit, and the connections between thenatural gas circuit and the recycled gas circuit serve only tocompensate the losses in the latter and to adjust its composition.

The installation of FIG. 1 comprises substantially cooling andliquefying exchangers, 10, 11, 12, 13, 14 and 15, common to the circuitsfor the natural gas to be liquefied and for the recycling gas, afractionating column 16, or degassing column, in which nitrogen isremoved from the liquefied mixture, and fractionating columns 82 and 92in which the heavy hydrocarbons (butane, propane) are separated.

The natural gas to be liquefied, available at 30° C. and at an absolutepressure of 50 bars absolute and having for example, the followingcomposition:

    ______________________________________                                                          Percent by volume                                           ______________________________________                                        Methane             85                                                        Ethane              7                                                         Propane             3                                                         Butanes             3                                                         Nitrogen and other light gases                                                                    2                                                         ______________________________________                                    

is introduced through a conduit 1 into the exchanger 10 where it iscooled to about -12° C., concurrently with the gas recycled at highpressure and a liquid fraction rich in butanes separated from thelatter, and in indirect heat exchange with the gas recycled at lowpressure. The cooled mixture is already partly liquefied and passesthrough the conduit 2 to an exchanger 11 where it is cooled to -48° C.in heat exchange with the recycled gas; its condensation continues.

The liquid-gas mixture which results is then passed through a conduit 3to an exchanger 12 where it is cooled down to -72° C.; it is thencompletely liquefied. It is then expanded in a valve 23 to about 7 barsabsolute pressure, mixed with the recycled gas supplied through aconduit 116 and introduced through a conduit 24 into a separator 25. Theliquid collected at the bottom of the separator passes through a conduit26 into an exchanger 13 where it is undercooled to about -85° C. andthen recombined through the conduit 27 and an expanding valve 28 withthe volatile recycled gas at the cold end of the exchanger 14. The gascoming from the separator 25 passes through a conduit 29 to theexchanger 13 where it is cooled to -97° C.; a substantial partliquefies. The mixture of liquid and vapor passes through the conduit 32into the separator 33. The liquid accumulating at the bottom of theseparator and comparatively rich in ethane passes through a conduit 34to an exchanger 15 where it is undercooled to about -127° C. It isremoved through a conduit 35. A fraction may be recombined through anexpanding valve 36 with the volatile gas rich in nitrogen issuing fromthe degasing tower 16. The other fraction passes through a conduit 37and an expanding valve 38 to 7 bars to the top of the degassing tower 16where nitrogen and other highly volatile gases are removed.

The gaseous fraction removed from the separator 33 through the conduit39 is again partly refrigerated and condensed in the reflux typeexchanger 14. The formed liquid consisting substantially of methane andcontaining little ethane is passed through the conduit 40 to theexchanger 15 where it is undercooled to about -127° C., then through aconduit 41 and the 7 bars expanding valve 42 into the upper part of thetower 16. The residual nonliquefied gas in the exchanger 14 is evacuatedthrough a conduit 43. A part is expanded near atmospheric pressure inthe valve 44 and recombined through a conduit 56 with the residualmixture of methane and nitrogen, to be evacuated prior to its reheating.The other part flows through the conduit 45 into the coil 46 located inthe sump of the degasing tower 16 where it is liquefied whilst heatingthe bottom of this column. It passes then through a conduit 47 into anexchanger 48, where it is undercooled concurrently with the naturalliquefied gas. This portion is evacuated through a conduit 49 and isthen divided into two fractions. The first fraction, expanded in a valve50 at about 7 bars, is passed in counter-current through the exchanger48 where it effects the undercooling of the liquid and then recombinesthrough the conduit 4 with the vapours discharged at the head of thedegasing tower 16. The second fraction passes through a conduit 51 intoan exchanger 52 where it is again undercooked, then in counter-currentthrough the same heat exchanger through a conduit 53 and an expandingvalve 54 to near atmospheric pressure. This produces the finalundercooling of the natural gas product. It is then evacuated through aconduit 55 to an exchanger 57 through which it passes after addition ofthe first part of the residual gas already mentioned through the conduit56. After reheating in this exchanger and then in the exchanger 86, towhich it is supplied through a conduit 58, in heat exchange with theheavier hydrocarbons (propane, butanes) which are to be added to theliquefied natural gas, it is removed through a conduit 59 and is used,for example, as fuel for boilers, owing to its high methane content.

The liquid introduced into the degasing tower 16 is here separated intoa liquid product free from nitrogen, withdrawn from the vessel through aconduit 60, and into vapours rich in nitrogen, removed from the topthrough a conduit 64. The liquid product is undercooled in the exchanger48. After the addition of heavier liquid hydrocarbons separated in thefractionating column 92 through the expanding valve 95, it is subjectedto a further undercooling in the exchanger 52. The liquefied andundercooled gas is finally expanded in the valve 63 to near atmosphericpressure for its introduction into the storage tank (not shown).

The vapours rich in nitrogen, taken from the top of the tower 16 throughthe conduit 64, are added through the conduit 4 to the fractionvaporized in the exchanger 48 and then through the expanding valve 36 toa fraction of the liquid coming from the separator 33 and undercooled inthe exchanger 15. The whole is then vaporized and reheated in theexchanger 15, leaves the same through a conduit 121, is added throughthe expanding valve 28 to the liquid recovered in the separator 25 andundercooled in the exchanger 13. The whole is vaporized and reheated inthe exchanger 14 and passes through the conduit 123 to the exchanger 13where it is further reheated. It flows then through the conduit 124 intothe exchanger 12, after addition through the expanding valve 114 of theliquid recovered in the separator 112 and undercooled in the exchanger12. The mixture of gas and liquid is vaporized and reheated in theexchanger 12, flows then through the conduit 126 to the exchanger 11,after addition through the expanding valve 108 and conduit 109 of theliquid recovered in the separator 106 and undercooled in the exchanger11. The mixture of gas and liquid is vaporized and reheated in theexchanger 11, passes then through the conduit 127 into the exchanger 10after addition through the conduit 89 of part of the liquid recovered inthe separator 75 and of liquid and gaseous fractions coming from thefractionating columns 82 and 92, the operation of which is describedfurther below. The mixture of gas and liquid is vaporized and heatednear ambient temperature in the exchanger 10 and is then evacuatedthrough the conduit 128.

The gaseous mixture formed in this way and reheated is then compressedby a turbo-compressor 70 to a pressure of about 30 bars. The compressedmixture passes through a conduit 71 to a cooler 72 equipped with a watercooling coil 73. A fraction consisting mainly of heavy hydrocarbons iscondensed and arrives with the remaining gas through the conduit 74 inthe separator 75.

The liquid rich in butanes, recovered at the bottom of the separator,passes through the conduit 76 into the exchanger 10 where it isundercooled and from which it is removed through the conduit 77. It isthen divided into two parts. A first part is expanded in the valve 83and combined with different other expanded liquid and gaseous fractions,whose origin will be discussed below, and is introduced therewith intothe recycled gaseous low-pressure mixture at the cold end of theexchanger 10. The second part is expanded in the valve 78 to about 15bars and then introduced through the conduit 79 into the fractionatingcolumn 82.

In the column 82, heated at the bottom by a steam coil 84, the liquidfraction is separated into comparatively volatile vapours and a residualliquid. The vapours pass through the conduit 85 to the exchanger 86where they are cooled in heat exchange with the residual mixture ofnitrogen and methane, and are then returned through the conduit 87, theexpanding valve 88 and the conduit 89 to the low pressure gas mixture atthe cold end of the exchanger 10.

The liquid removed from the bottom of the column 82 through the conduit90 is expanded in the valve 91 to about 12 bars and introduced into thefractionating column 92. In this column, equipped at the bottom with asteam coil 31 and at the head with a circulating water cooler 101, aliquid fraction rich in butanes is separated, withdrawn through theconduit 96 and, after cooling in the cooler 97, equipped with a watercoil 98, passed through the conduit 99 to the expanding valve 100, thenthrough the conduits 30 and 89 to the cold end of the exchanger 10.

The more volatile fraction, rich in propane, condensed in the cooler 101is evacuated through the conduit 93. It is undercooled in the exchanger86 by heat exchange with the residual nitrogen-methane mixture and flowsthen through the conduit 94 to the exchanger 57 where it is cooled againby the same mixture. It is then expanded in the valve 95 to the pressureof 7 bars and recombined with the liquid methane at the cold end of theexchanger 48.

The residual gas withdrawn from the head of the column 92 is combinedthrough the conduit 102 and the expanding valve 103 to 7 bars to theliquid and gaseous low pressure fractions recycled at the cold end ofthe exchanger 10.

The gas mixture remaining in the separator 75 passes through the conduit104 into the exchanger 10 where it is cooled and partly condensed, thenthrough the conduit 105 to the separator 106. The liquid fraction richin butanes and propane recovered from the separator passes through theconduit 107 to the exchanger 11 where it is undercooled, then expandedin the valve 108, and recombined through the conduit 109 with thegaseous mixture recycled at low pressure at the cold end of theexchanger 11.

The gas mixture remaining in the head of the separator 106 passesthrough the conduit 110 into the exchanger 11 and leaves the samethrough the conduit 111. A liquid fraction rich in propane and ethane isrecovered in the separator 112. This fraction is passed through theconduit 113 into the exchanger 12 and after being undercooled in thesame, is expanded in the valve 114 and added to the low pressure gasmixture at the cold end of the exchanger 12.

Finally, the residual gas withdrawn from the head of the separator 112is introduced through the conduit 115 into the exchanger 12 where it ispartly liquefied and then added through the conduit 116 to the liquefiednatural gas expanded in the valve 23 and introduced together therewiththrough the conduit 24 into the separator 25.

The installation of FIG. 2 comprises substantially the cooling andliquefying exchangers 10, 11, 12, 13 and 14, common to the natural gasand recycling gas circuits which are here completely separate, the tower16 for extracting nitrogen from the liquefied natural gas and thefractionating columns 209, 293 and 297 for separating from the naturalgas a fraction of heavy hydrocarbons before combining them with theliquid methane.

The natural gas to be liquefied, available at ambient temperature and ata pressure of about 50 bars, having a composition similar to thatindicated above and containing also some C₅ and C₆ hydrocarbons, arrivesthrough the conduit 201 in the exchanger 10 where it is cooled to -12°C. and slightly condensed. It passes then through the conduit 205 into aseparator 206. The condensate enriched in heavy hydrocarbons, passesthrough the conduit 207 and the expansion valve 208 to 15 bars to thehead of a rectification column 209.

The column 209, heated at the bottom by a steam coil 291, separates theintroduced liquid fraction into methane and ethane vapours and a liqudresidual. The vapours withdrawn from the head through the conduit 305are passed to an exchanger 282 where they are cooled by exchange with aresidual methane and nitrogen mixture, whose origin will be explainedfurther below. Withdrawn therefrom through conduit 309, they are thenexpanded to about 10 bars absolute in the valve 317 and after additionof heavier hydrocarbons (propane, butane, C₅ hydrocarbons) are liquefiedand undercooled in the exchanger 280 in exchange with the same residualmixture, and finally are added through the conduit 320 to the liquidmethane coming from the degasing tower 16.

One may divert a liquefied fraction of ethane and methane upstream ofthe expanding valve 317 and expand the same in a valve 310 to about 7bars and add it through conduits 311, 312, and 241 to other hydrocarbonsfractions which are recycled to the cold end of the exchanger 10.

The residual liquid separated in the sump of the column 209 is expandedin the valve 292 and introduced into a centre zone of the fractionatingcolumn 293, equipped with a water condenser and heated by a steam coil294 which separates it into propane and lighter hydrocarbons on the onehand, and a fraction rich in butanes and heavier hydrocarbons on theother hand.

The propane condensed in the condenser 295 at the head of the column 293is withdrawn through the conduit 318, then expanded in the valve 319 to10 bars and added through the valve 304 to C₅ liquid hydrocarbons (andheavier) then added to the liquid methane and ethane mixture alreadymentioned at the warm end of the exchanger 280 for being undercooled andcombined with the liquefied methane.

The uncondensable vapours coming from the head of the column 293 throughthe conduit 306 are expanded in the valve 307 to 7 bars and combinedwith the other recycled fractions at the cold end of the exchanger 10through the conduit 241.

The liquid rich in butanes, taken from the bottom of the column 293, isexpanded to 10 bars in the valve 296 and introduced into thefractionating column 297. This separates the butanes and lighterhydrocarbons from the C₅ and heavier hydrocarbons (benzoles). It hasalso a steam coil 298 for heating the sump and a water condenser 299.

The butanes condensed in the cooler 299 are withdrawn through theconduit 313 and expanded in the valve 314 to about 7 bars, then afteradding through the expanding valve 315 the uncondensable gases comingfrom the head of the column 297, combined through the conduits 316, 312and 241 with the other recycled fractions at the cold end of theexchanger 10 to compensate for inevitable butane and propane losses inthe cycling gas.

The C₅ and heavier hydrocarbons withdrawn from the base of the column297 pass through the conduit 300 to a cooler 301 with a circulatingwater coil 302 and after undercooling therein are combined through theconduit 303 and the valve 304 with the liquid propane fraction at thewarm end of the exchanger 280.

The remaining natural gas evacuated from the separator 206 through theconduit 210 is further cooled in the exchanger 11 to about -47° C.,passes then through the conduit 211 into the exchanger 12 where it iscooled to about -78° C., and liquefied. It then flows through theconduit 212 into the exchanger 13 where it is undercooled to -105° C. Itpasses then through the conduit 213 to the coil 214 arranged in the sumpof the degasing tower 16 where it is undercooled and heats the column.It is finally expanded in the valve 215 to about 10 bars and introducedin to the centre part of this column.

In addition to the heating coil 216, the column 16 is equipped with acondenser 267 cooled by vaporizing a nitrogen-methane mixture boiling at-140° C. at 7 bars and coming from the closed cooling cycle. Theliquefied natural gas is here separated into a liquid fractionpractically free from nitrogen and nitrogen-rich vapours.

The liquid withdrawn from the base of the column 16 is passed throughthe conduit 220 to the sub-cooling exchanger 221. After addition of thepreviously separated and then undercooled heavier hydrocarbons throughthe conduit 320, the liquid is undercooled to about -161° C. in theexchanger 221 in exchange of heat with the methane-nitrogen cyclemixture and then expanded in the valve 222 at the inlet of the storagetank (not shown).

The methane and nitrogen vapours uncondenssed in the condenser 267 ofthe tower 16 are extracted through the conduit 227 and passedsuccessively through the conduits 278 and 281 to the exchangers 280 and282 where they are reheated to near ambient temperature, then expandedin the valve 283 and removed through the conduit 284 for use, forexample, as fuel.

In view to reduce the nitrogen content of the cycling gas it is possibleto add to these vapours through the expanding valve 270 a fraction ofthe gas coming from the separator 262 prior to its entry into theexchanger 14. Similarly, in order to increase the nitrogen content inthe cycling gas, it is possible to remove a fraction of these methaneand nitrogen vapours, to expand them in the valve 285 and to add them tothe gas mixture coming from the condenser 267 of the degasing towerprior to vaporization and reheating in the exchanger 14.

The circuit of the gas mixture acting as refrigerating fluid is asfollows:

The gas mixture is compressed by a turbo-compressor 230 to a pressure ofabout 30 bars absolute. It is then cooled in the cooler 231 with acirculating water coil 232 to about +30° C. and passed through theconduit 233 to the separator 244.

The liquid recovered in the separator is introduced through the conduit235 into the exchanger 10 where it is undercooled to about -12° C. Afterevacuation through the conduit 236, it is combined through the expandingvalve 239 at 6 bars and the conduits 240 and 241 with the low-pressurerecycled gas arriving through the conduit 289 at the cold end of theexchanger 10 for being vaporized so as to contribute to the coolingeffect of this exchanger. However, a fraction may be passed through theexpanding valve 237 and the conduit 238 to the fractioning column 209for regulating the composition of the gas mixture of the refrigeratingcycle, if the same contains too much high boiling products.

The gas mixture of the cycle leaves the separator 234 through theconduit 242, is then cooled in the exchanger 10 to about -12° C. andundergoes partial condensation. It passes then through the conduit 243to the separator 244. The liquid fraction recovered in the separatorpasses through the conduit 245 to the exchanger 11 where it isundercooled to -47° C. and is then combined through the conduit 246 andthe expanding valve 247 with the low pressure recycling gas at the coldend of the exchanger 11.

The cycling gas remaining passes through the conduit 248 to theexchanger 11 where it is cooled to -47° C. and again partly condensed,and then through the conduit 249 to the separator 250. The recoveredliquid fraction flows from this separator through the conduit 251 to theexchanger 12 where it is undercooled to about -78° C., is then combinedthrough the conduit 252 and the expanding valve 253 to the coldlow-pressure cycling gas at the cold end of the exchanger 12.

The cycling gas removed from the head of the separator 250 through theconduit 254 is cooled again to -78° C. and partly condensed in theexchanger 12. It is introduced through the conduit 255 into theseparator 256. The liquid fraction recovered in this separator passesthrough the conduit 257 to the exchanger 13 where it is undercooled toabout -105° C. and then combined through the conduit 258 and theexpanding valve 259 with the low-pressure cold recycling gas at the coldend of the exchanger 13.

The remaining cycling gas evacuated from the head of the separator 256through the conduit 260 is cooled to -105° C. and mainly condensed inthe exchanger 13, and then passed through the conduit 261 to theseparator 262. The liquid recovered in this separator is introducedthrough the conduit 263 into the exchanger 14 where it is undercooled toabout -132° C. and then through the conduit 264, the expanding valve 265and the conduit 266 to the condenser 267 at the head of the degasingtower 16, after addition through the conduit 275 of anothermethane-nitrogen fraction, the origin of which will be explained below.

The residual gas from the separator 262 is evacuated through the conduit269 towards the exchanger 14 where it is cooled to about -132° C. andcondensed. The liquid flows through the conduit 272 to the exchanger221. It is first undercooled concurrently with the liquefied naturalgas, then raised in countercurrent through the conduit 273 and theexpansion valve 274 to 7 bars through the same exchanger, ensuring theundercooling of both these liquids to about -160° C. After partialvaporization it is added through the conduit 275 to the liquidintroduced through the conduit 266 into the condenser 267 of thedegasing tower 16.

The nitrogen-methane mixture already partly vaporized in the condenser267 and evacuated through the conduit 268 flows to the cold end of theexchanger 14 and is totally vaporized and reheated. The gaseouslow-pressure recycling mixture is then introduced successively into theexchangers 13, 12, 11, and 10 through the conduits 286, 287, 288 and289, after addition of condensed and undercooled liquid fractions comingfrom the separators 256, 250, 244 and 234 as indicated above. Afterwarming up to ambient temperature the gaseous cycling mixture passesthrough the conduit 290 to the turbo-compressor 230.

What we claim is: .[.1. In a method for cooling a gaseous mixture to lowtemperature and producing at least one constituent of this mixture inliquid phase, comprising subjecting a first gaseous mixture to afractionate condensation in a plurality of heat-exchange stages with atleast one of the condensed fractions being expanded to a first pressureand vaporized in heat exchange with said first gaseous mixtureundergoing fractionate condensation, expanding to a second pressure saidat least one constituent after its condensation and withdrawing saidexpanded constituent as product, said first pressure being substantiallyhigher than said second pressure, thereafter recompressing said at leastone fraction to a third pressure substantially higher than said firstpressure, and recombining said recompressed fraction with said firstgaseous mixture; the improvement comprising progressively cooling asecond gaseous mixture in a plurality of heat-exchange stages with saidfirst mixture, said second mixture having the same main components assaid first mixture, and maintaining said first gaseous mixture with saidsecond gaseous mixture separate at least until they are under conditionsof temperature and pressure such that said constituent to be produced inliquid phase is at least mostly condensed within said second gaseousmixture and said at least one condensed fraction has been withdrawn fromsaid first gaseous mixture for expansion and vaporization..]. .[.2. Amethod as claimed in claim 1, said second gaseous mixture beinginitially at a fourth pressure substantially higher than said thirdpressure..]. .[.3. A method as claimed in claim 1, in which said firstgaseous mixture flows within a closed cycle and all the expanded andvaporized fractions separated from said first gaseous mixture arereunited to reconstitute said first gaseous mixture..]. .[.4. A methodas claimed in claim 3, in which said second gaseous mixture is subjectedto a fractionate condensation, and adding at least a part of a fractionseparated from said second gaseous mixture during fractionatecondensation to an expanded fraction of said first gaseous mixtureundergoing vaporization, thereby to compensate for losses in said closedcycle and to adjust its composition..]. .[.5. A method as claimed inclaim 3, and adding at least a fraction separated during the fractionatecondensation of said first gaseous mixture to a fraction separated fromsaid second gaseous mixture at such a flow rate as to adjust thecomposition of said first gaseous mixture..]. .[.6. A method ofutilizing a first gaseous mixture at relatively low pressure torefrigerate a second gaseous mixture at relatively high pressure,comprising establishing a refrigeration cycle in which said firstgaseous mixture is subjected to fractionate condensation in a pluralityof heat exchange stages with at least one of the condensed fractionsbeing expanded and vaporized in heat exchange with said first gaseousmixture undergoing fractionate condensation, then recompressed to saidrelatively low pressure and reunited with said first gaseous mixture,and progressively cooling said second mixture in a plurality of saidheat exchange stages..]. .[.7. A method as claimed in claim 6, andcombining said first and second gaseous mixtures downstream of aplurality of said heat exchange stages..]. .Iadd.8. A method for coolinga second gaseous mixture to low temperature and producing at least oneconstituent of said mixture in liquid phase, comprising:a. cooling andsubjecting a first gaseous mixture containing at least one component ofsaid second gaseous mixture, to a fractionate condensation under a firstpressure, b. expanding at least two condensed fractions obtained duringthe fractionate condensation of said first gaseous mixture, reunitingthe expanded condensed fractions with said first gaseous mixture under asecond pressure lower than said first pressure, vaporizing and reheatingthe reunited fractions with said first gaseous mixture under said secondpressure, by heat exchange with said second gaseous mixture undergoingcooling, and with said first gaseous mixture undergoing fractionatecondensation, and recompressing said first gaseous mixture to said firstpressure, c. reuniting under said first pressure said second gaseousmixture undergoing cooling with said first gaseous mixture undergoingfractionate condensation, when said gaseous mixture is in conditions oftemperature and pressure such that a major portion of said constituentto be produced in liquid phase is condensed within said second gaseousmixture, and after the first condensed fraction of said first gaseousmixture has been withdrawn for expansion and vaporization, continuingthe fractionate condensation of the mixture so obtained under said firstpressure until there is obtained a last condensed fraction containing amajor part of said constituent to be produced in liquid phase, d.expanding to said second pressure the last condensed fraction, andseparating the last expanded fraction into a liquid portion expanded toa pressure lower than said second pressure and a residual gaseousportion for recompression with said first gaseous mixture, andwithdrawing said last expanded liquid portion as a product stream, ande. prior to reuniting said first gaseous mixture and said second gaseousmixture, condensing at least partially said second gaseous mixture undera third pressure which is higher than said first pressure, and expandingsaid second gaseous mixture at least partially condensed to said firstpressure. .Iaddend. .Iadd.9. In a method for cooling a second gaseousmixture to low temperature and producing at least one constituent ofsaid mixture in liquid phase, comprising:a. cooling said second gaseousmixture until at least a major portion of said consituent to be producedin liquid phase is condensed within said second gaseous mixture, thenexpanding at least said condensed constituent to a first pressure, b.cooling and subjecting a first gaseous mixture comprising at least onecomponent of said second gaseous mixture, to a fractionate condensationunder a second pressure which is higher than said first pressure, c.expanding at least two condensed fractions obtained during thefractionate condensation of said first gaseous mixture, reuniting theexpanded fractions with said first gaseous mixture under a thirdpressure which is intermediate said first and second pressures,vaporizing and reheating the reunited fractions with said first gaseousmixture under said third pressure by heat exchange with said secondgaseous mixture undergoing cooling and with said first gaseous mixtureundergoing fractionate condensation and recompressing said first gaseousmixture to said second pressure, the improvement comprising adjustingthe composition of said first gaseous mixture in at least one component:d. when there is an insufficient quantity of said component, subjectingsaid second gaseous mixture undergoing cooling to fractionatecondensation, rectifying at least a part of a condensed fraction of saidsecond gaseous mixture under at least a pressure intermediate saidsecond and third pressure, adding at least a part of a fractionseparated during said rectification to said first gaseous mixture, ande. when there is an excess of said constituent, rectifying at least apart of a condensed fraction of said first gaseous mixture under atleast a pressure intermediate said second and third pressures, andadding at least a part of a fraction separated during said rectificationto said second gaseous mixture.